1. Field of the Invention
The present invention is related to a dual current-limiting circuit for a dc brushless motor and more particularly to a current-limiting circuit for operating a dc brushless motor while locked or in a failure condition, and to utilizing a dual current-limiting method to tremendously reduce current passing through two ends of a coil, so as to limit current and provide a low voltage level which is adequate for restarting the motor when the failure condition is eliminated.
2. Description of the Related Art
When a motor is locked or motor speed is decreased due to dust and dirt, both the motor current passing through a coil and heat produced thereon may tremendously increase. The insulated cover of the coil is heated, thereby resulting in deterioration and cracking due to high temperature. In order to prevent great current from passing through the coil while the fan motor is locked or fails to operate in a normal manner, an auto restart function has been built in a drive circuit of the motor. When the fan motor is operated abnormally, the drive circuit generates time-intermittent signals to allow current passing through the coil short-term (a few seconds) for avoiding great current continuously passing therethrough. If abnormal operation of the fan motor cannot still be eliminated after the current continuously passes therethrough short-term, it interrupts the current to pause the operation. After interrupting a short-term currently supply, an auto restart circuit restarts the motor fan to allow current to pass through the coil.
The restart function of the fan motor is periodically repeated as long as abnormal operation is not eliminated. It is available only for low airflow or low power consumption fan motors since the restart function, which uses maximum current manner, may consume great pulse current for restarting.
FIG. 1 illustrates a schematic view of a conventional driver circuit for a single phase dc brushless motor.
Referring to FIG. 1, the driver circuit for the single phase dc brushless motor includes four transistors Q1, Q2, Q3, Q4, five resistors R1, R2, R3, R4, R5, a coil L1, a Hall element IC1, a driver element IC2, and a capacitor C1. The Hall element IC1 is used for detecting changes in a permanent magnetic field of a rotor to thereby output a weaker Hall voltage. And then the driver circuit IC2 magnifies the Hall voltage to further output alternative signals (square waveform) from two terminals O1 and O2. The coil L1 is alternatively actuated to change current direction, so as to rotate a rotor.
FIG. 2 illustrates a voltage diagram for time of the conventional driver circuit for the single phase dc brushless motor.
Referring to FIG. 2, the driver element IC2 has a restart function. The restart function of the driver element IC2 may be turned off when the motor is operated in a normal manner and turned on when the motor is operated in an abnormal manner, so as to allow a great current to pass through the coil L1 for only a short-term (a few seconds). When the motor is restarted by means of the restart function of the driver element IC2, the motor is driven by maximum current and thus it is available only for a low airflow motor or low power consumption motor. By contrast, a high airflow motor or high power consumption motor operating abnormally may endure great currents of more than 10 amperes to form a high pulse which results in damage to the transistor of the electronic components. The transistor cannot endure a great current or high pulse even if the driver element allows great current to pass through the coil for only a short-term.
The present invention intends to provide a dual current-limiting circuit that instantly turns on a first switch of a first limiting circuit adapted to suppress a voltage signal output to a coil while inputting a great abnormal input current, and subsequently turn on a second switch of a second limiting circuit adapted to suppress current passing there through at a low voltage level which is inadequate to damage the driver circuit. The dual current-limiting circuit is used to prevent the electronic components from damage due to inputting great abnormal input current in such a way as to mitigate and overcome the above problem.
The primary objective of this invention is to provide a dual current limiting circuit for a dc brushless motor which instantly turns on a first switch of a first limiting circuit adapted to supply a first voltage signal to a coil while limiting a great abnormal input current, and subsequently turned on a second switch of a second limiting circuit adapted to also suppress current passing therethrough but at a low voltage level which is inadequate to damage the driver circuit and still adequate to restart the motor if the great abnormal input current is eliminated.
The secondary objective of this invention is to provide a dual current-limiting circuit for a dc brushless motor which is adapted to suppress great current while auto-restarting, so as to avoiding instant great current or high pulses from passing through a coil and thereby being available for a high airflow fan motor.
The present invention is a dual current-limiting circuit for a dc brushless motor. The dual current-limiting circuit includes an over current detective circuit, an operation detective circuit, and a current limiting circuit. The over-current detective circuit produces a voltage signal that is used to determine whether an over current has been input to the motor. The operation detective circuit also produces a voltage signal used to determine whether the motor is operating in a normal manner. The current-limiting circuit decides to turn on a first switch of a first current limiting circuit and to subsequently turn on a second switch of a second current-limiting circuit depending upon the two voltage signals. When the second switch is turned on, the current-limiting circuit is capable of suppressing current passing through a coil at low voltage level, which is inadequate to damage a driver circuit and still adequate to restart the motor if the great abnormal input current is eliminated.
Other objectives, advantages and novel features of the invention will become more apparent from the following detailed description and the accompanying drawings.